1. Field of the Invention
The present invention relates to an amplitude conversion circuit for converting an amplitude of a signal, and more particularly, to an amplitude conversion circuit including transistors that have the same conductivity type.
2. Description of the Background Art
There is widely known an amplitude conversion circuit (level converting circuit) for converting a voltage level and an amplitude of a signal. For example, Japanese Patent Application Laid-Open No. 2005-12356 discloses the level converting circuit including transistors that have the same conductivity type. The transistors of a single conductivity type are used in this manner, which leads to a simplification of the manufacturing process as well as a cost reduction.
For example, in the level converting circuit of FIG. 1 of Japanese Patent Application Laid-Open No. 2005-12356, an input signal IN is supplied to a gate of an input transistor 6 through capacitive coupling of a capacitance element Cc. A source of the input transistor 6 is fixed at a constant power supply potential −VL, and thus a gate-source voltage of the input transistor 6 changes with the amplitude equal to that of the input signal IN. Further, in the level converting circuit, a resistance element 7 is connected between the gate and the source of the input transistor 6, whereby the gate-source voltage becomes zero when the input signal IN is at the inactive level in a steady state. Accordingly, in the level converting circuit, the amplitude of the input signal IN needs to be equal to or larger than a threshold voltage of the input transistor 6 in order that the input transistor 6 turns on/off in accordance with a level of the input signal IN.
Nowadays, display devices have come into use, which have a configuration in which pixels composed of display elements such as liquid crystal elements and organic electroluminescence (EL) elements and drive circuits thereof are formed on the same insulating substrate, to thereby achieve a cost reduction of a display device. In the formation of the transistors used in the above-mentioned pixels and drive circuits, a manufacturing process for, for example, low temperature poly-silicon or amorphous silicon is used. A threshold voltage of a transistor formed through the above-mentioned manufacturing process is approximately 3 V to 5 V, including variations. On the other hand, a control signal input to the display device is generated by a semiconductor integrated circuit, and a level thereof is typically from 3 V to 3.3 V.
When the threshold voltage of the transistor used in the display device is equal to an amplitude of the control signal as described above, it is difficult to cause the drive circuit to operate in response to a control signal thereof. Therefore, an amplitude conversion circuit composed of a semiconductor integrated circuit is provided in a conventional display device, which amplifies control signals. This enables normal operation of the drive circuit, but a cost increase of a display device is caused due to the addition of an amplitude conversion circuit composed of a semiconductor integrated circuit.
In the case of the amplitude conversion circuit configured with only transistors of a single conductivity type, such as the level converting circuit of Japanese Patent Application Laid-Open No. 2005-12356, costs may be reduced because the amplitude conversion circuit can be formed on the same insulating substrate as the pixels and drive circuits. However, an input signal having a threshold voltage of its input transistor or higher voltage is required, which makes it difficult to use the amplification conversion circuit for amplifying control signals generated by the semiconductor integrated circuit.